cDNA clones corresponding to the major proteins expressed in mouse epidermis have been isolated and characterized. Using a combination of in situ hybridization with RNA probes (which are specific for individual mRNAs) and indirect immunofluorescence with monospecific antisera (which were elicited with synthetic peptides corresponding to unique sequences within each protein), it is possible to show that these genes belong to at least four subsets: those expressed predominantly in the proliferating basal layer of the epidermis; those expressed predominantly in the differentiated suprabasal spinous layers and, to a lesser extent, in the granular layer; those only expressed in the granular layer; and those only expressed under hyperproliferative conditions. Genes representing each subset have been isolated and sequenced. Various strategies have been employed to identify sequences regulating the expression of these genes, including vector constructs using different regions of the genomic clones to drive expression of the chloramphenicol acetyltransferase gene and the production of transgenic mice which express a human differentiation-associated keratin gene in a tissue- and developmental-specific pattern. A gene encoding a cysteine-rich protein' which is a major component of the cornified envelope, has been isolated and shown by in situ hybridization experiments to be expressed in the granular layer of the epidermis. A monospecific antiserum has been used to demonstrate that the C- terminal portion of this protein is only detectable on the inner surface of mature envelopes. Monospecific antisera that have been produced against mouse and human keratins and other epidermal- specific differentiation products have been used to study various stages of carcinogenesis. gene expression in mutant mice exhibiting developmental defects in epidermal differentiation the induction of terminal differentiation in malignant cell lines by pharmacological agents, the in vivo kinetics of expression of the differentiation-associated keratins with respect to cell division, and requirements for the induction of terminal differentiation products in vitro.